1. Field of the Invention
This invention relates to a method for adjusting a threshold value of a vehicle-mounted device employed in an automatic toll-collecting system to perform automatic collection of a traffic toll, a vehicle-mounted device structured so as to perform adjustment of the threshold value thereof, and a threshold-value adjusting device for performing the threshold-value adjustment.
2. Description of Related Art
Various automatic toll-collecting systems have been proposed for performing automatic collection of a traffic toll via transmission of radio waves between a roadside device and a vehicle-mounted device.
The vehicle-mounted device employed in this automatic toll-collecting system is activated solely within a communication area formed by the roadside device, so that communication with the roadside device is completed within this communication area, to suppress communication interference and reduce power consumption. Specifically, the vehicle-mounted device is provided with an electric field-strength detecting circuit to detect the strength of radio waves emitted from the roadside device. Further, the device is designed to be activated and begin communication with the roadside device when the field-strength value reaches a predetermined threshold value, to ensure sufficient communication quality.
However, when fluctuations occur in the above-described threshold value, the quality of communication declines, and power consumption of the vehicle-mounted device becomes large. For example, when the threshold value is set higher than an appropriate value, the timing with which entry of the vehicle-mounted device into the communication area is detected is delayed, and the communication area is shortened. When the communication area is shortened in this way, the time with which the vehicle-mounted device can communicate with the roadside device is shortened. Therefore, cases wherein communication is not completed may occur. Additionally, when the threshold value is set at an extremely high value, the communication area may not be detected, even when the vehicle-mounted device passes under the roadside device. Thus, communication with the roadside device cannot be performed.
Conversely, when the threshold value is set lower than the appropriate value, background-noise radio waves are detected, and the vehicle-mounted device may be activated even at a location outside the communication area. Therefore,there is a chance that communication interference may occur. Additionally, power consumption of the vehicle-mounted device increases, and, in a case wherein a dry-cell battery is employed as the power source of the vehicle-mounted device, the battery may be expended prematurely.
In this regard, a method may be considered wherein the threshold value of the field-strength detecting circuit is set with a variable resistor. The resistance of the variable resistor is adjusted manually, and threshold-value adjustment is performed so that each vehicle-mounted device can detect the set field strength. However, fine adjustment of the sensitivity of the field-strength detecting circuit is necessary, thereby representing a significant design problem. Furthermore, this sensitivity adjustment must be performed before case assembly, and so the possibility exists that sensitivity may be affected by the case after case assembly.